Fly Levelling

Fly levelling is a process of finding the level difference between two points and the levelling consists of taking back sights and fore sights only and not intermediate sights. Differential levelling is the determination of level difference between two points; check levelling is finding the level difference between points in a way of checking the accuracy of levelling works already done; and fly levelling is the procedure of accomplishing the objective of finding out the level difference between two points, which are far apart. It can also be used for transferring a bench mark.

Fly levelling procedure

1. Set up the level on a firm ground and do the temporary adjustments. The instrument should be set up approximately midway between the change points.
2. Direct the telescope towards the staff, which is held vertically on the point.
3. Focus the telescope.
4. Bring the staff between the two vertical hairs.
5. Check the bubble. If it is not in the exact centre, use the screw in line with the telescope and bring it to the centre.
6. Read the staff when the horizontal hair of the diaphragm appears to cut it and record the correct reading.
7. Take the first reading on the bench mark and enter the reading in the back sight column of the field book.
8. Take fore sight reading on the change point, if the second bench mark is far away, and enter the reading in the fore sight column of the next horizontal line.
9. Shift the instrument and do all the temporary adjustments.
10. Take back sight on the same change point and enter the reading in the back sight column of the same horizontal line.
11. Take fore sight reading on another change point, if the next bench mark is far away, and enter the reading in the fore sight column of the next horizontal line.
12. Shift the instrument near to the next bench mark and again take readings on the change point and enter it in the back sight column of the same horizontal line.
13. Repeat the above process for a number of bench marks.
14. If possible close the fly level at the starting point.
15. Record all the reading and data in the field book.